It is known that the obtention of high purity oxides in a finely divided form constitutes an important research objective in different technical fields, to prepare for example ceramics, abrasives, catalyst supports, etc. . . .
It is known that rare earths are present in numerous minerals and constitute by-products of the preparation of various metals. Their utilization poses difficult technical and economic problems. In effect, the conventional methods of the processing of oxide particles generally employ either precipitation processes or those of the sol-gel type. In precipitation methods, the addition of the precipitating agent leads to contamination and thus a change in the purity of the oxide obtained. In the sol-gel methods, processing is generally long and the basic products used are expensive.
Furthermore, the calcination processes which generally represent the final stage in the obtention of oxides introduce the risk of additional pollution by the environment and lead to particles the surface whereof are less than favorable for certain applications.
In U.S. Pat. No. 3,098,708, the obtention of particles of oxides of thorium and uranium by a hydrothermal process consisting of heating an acid solution such as a hydrochloric or sulfuric acid solution of oxides of these metals to a temperature of 170.degree. to 250.degree. C., under a pressure of 100 to 400 psi (approximately 6.5.times.10.sup.5 to 2.6.times.10.sup.6 Pa), is described. In this patent, it is indicated that the process is of interest in particular because the rare earths potentially present do not precipitate during the carrying out of this hydrothermal process.